Adaptive transmission mode switching

ABSTRACT

A node ( 28 ) of a radio access network ( 20 ) communicates over a radio interface ( 32 ) with a wireless terminal ( 30 ). The node ( 28 ) comprises a transmitter ( 34 ) and a controller ( 40 ). The transmitter ( 34 ) selectively operates in plural multiple input multiple output (MIMO) modes for downlink transmission over the radio interface ( 32 ). The controller ( 40 ) uses both a terminal speed value and a throughput value to make a determination when to switch between the plural multiple input multiple output (MIMO) modes for communicating with the wireless terminal. The plural MIMO modes comprise a first mode and a second mode. In the first mode open loop MIMO operates with cyclical diversity delay. In the second mode open loop MIMO operates without cyclical diversity delay. Although operating in open loop MIMO, advantages such as those of closed loop MIMO are realized.

TECHNICAL FIELD

The technology relates to wireless telecommunications, and particularlyto the transmission of information using multiple antennas, includingmultiple input and multiple output (MIMO) transmission modes.

BACKGROUND

In a typical cellular radio system, wireless terminals (also known asmobile stations and/or user equipment units (UEs)) communicate via aradio access network (RAN) to one or more core networks. The radioaccess network (RAN) covers a geographical area which is divided intocell areas, with each cell area being served by a base station, e.g., aradio base station (RBS), which in some networks may also be called, forexample, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographicalarea where radio coverage is provided by the radio base stationequipment at a base station site. Each cell is identified by an identitywithin the local radio area, which is broadcast in the cell. The basestations communicate over the air interface operating on radiofrequencies with the user equipment units (UE) within range of the basestations.

In some versions of the radio access network, several base stations aretypically connected (e.g., by landlines or microwave) to a controllernode (such as a radio network controller (RNC) or a base stationcontroller (BSC)) which supervises and coordinates various activities ofthe plural base stations connected thereto. The radio networkcontrollers are typically connected to one or more core networks.

The Universal Mobile Telecommunications System (UMTS) is a thirdgeneration mobile communication system, which evolved from the secondgeneration (2G) Global System for Mobile Communications (GSM). UTRAN isessentially a radio access network using wideband code division multipleaccess for user equipment units (UEs). In a forum known as the ThirdGeneration Partnership Project (3GPP), telecommunications supplierspropose and agree upon standards for third generation networks and UTRANspecifically, and investigate enhanced data rate and radio capacity.Specifications for the Evolved Universal Terrestrial Radio AccessNetwork (E-UTRAN) are ongoing within the 3^(rd) Generation PartnershipProject (3GPP). The Evolved Universal Terrestrial Radio Access Network(E-UTRAN) comprises the Long Term Evolution (LTE) and SystemArchitecture Evolution (SAE). Long Term Evolution (LTE) is a variant ofa 3GPP radio access technology wherein the radio base station nodes areconnected to a core network (via Access Gateways, or AGWs) rather thanto radio network controller (RNC) nodes. In general, in LTE thefunctions of a radio network controller (RNC) node are distributedbetween the radio base stations nodes (eNodeB's in LTE) and AGWs. Assuch, the radio access network (RAN) of an LTE system has an essentially“flat” architecture comprising radio base station nodes withoutreporting to radio network controller (RNC) nodes.

Recently cellular operators have begun offering mobile broadband basedon the Long Term Evolution (LTE) wireless system. Fuelled by new devicesdesigned for data applications, the end user performance requirementsare steadily increasing. Consequently, techniques that enable cellularoperators to utilize their spectrum resources more efficiently are ofincreasing importance. Table 1 shows various downlink transmission modesfor Long Term Evolution (LTE), including various MIMO modes.

TABLE 1 LTE Transmission Modes Transmission Mode Downlink TransmissionScheme Mode 1 Single Antenna Port (SISO or SIMO) Mode 2 TransmitDiversity Mode 3 Open-Loop Spatial Multiplexing Mode 4 Closed-LoopSpatial Multiplexing Mode 5 Multi-User MIMO Mode 6 Closed-Loop Rank-1Spatial Multiplexing Mode 7 Single Antenna Port Beamforming Mode 8Dual-Layer Beamforming

MIMO (multiple input and multiple output) is one of the key technologiesthat provides substantial improvements in spectral efficiency. MIMOinvolves the use of multiple antennas at both the transmitter andreceiver to improve communication performance. MIMO offers significantincreases in data throughput and link range without additional bandwidthor increased transmit power. To do so, MIMO spreads the same totaltransmit power over the antennas to achieve an array gain that improvesthe spectral efficiency (more bits per second per hertz of bandwidth) orto achieve a diversity gain that improves the link reliability (reducedfading).

Currently, there are two widely used MIMO techniques: OL-MIMO (open loopMIMO) and CL-MIMO (closed loop MIMO). As summarized in a white paperentitled “Maximizing LTE Performance Through MIMO Optimization”(http://rfsolutions.pctel.com/artifacts/MIMOWhitePaperRevB-FINAL.pdf),open loop and closed loop modes differ in the level of detail andfrequency with which channel conditions are reported by the UE. TheeNodeB relies on detailed and timely information from the UE in order toapply the best antenna and data-processing techniques for the existingchannel conditions. Depending on the UE's data-processing speed as wellas the quality of its connection to the eNodeB in both uplink anddownlink, LTE will operate in either closed loop or open loop mode.

The eNodeB communicates with a UE in open loop when the UE is moving toofast to provide a detailed report on channel conditions in time for theeNodeB to select a precoding matrix. Other factors, such as UEprocessing speed or uplink data capacity (which may also be affected byUE specifications), may result in open loop operations even when the UEis moving relatively slowly. The UE's capabilities are therefore crucialfor achieving the best results from particular multipath conditions. Inopen loop operations, the eNodeB receives minimal information from theUE: (1) a Rank Indicator (RI) which indicates the number of layers thatcan be supported under the current channel conditions and modulationscheme; and (2) a Channel Quality Indicator (CQI) which is a summary ofthe channel conditions under the current transmission mode, and whichroughly corresponds to the signal to noise ratio (SNR). The eNodeB thenuses the CQI to select the correct modulation and coding scheme for thechannel conditions. Combined with this modulation and coding scheme, CQIcan also be converted into an expected throughput. The eNodeB adjustsits transmission mode and the amount of resources devoted to the UEbased on whether the CQI and RI reported by the UE matches the expectedvalues, and whether the signal is being received at an acceptable errorrate.

In closed loop operations, the UE analyzes the channel conditions ofeach transmission (Tx), including the multipath conditions. In theclosed loop MIMO, the receiver reports channel status to the transmittervia a special feedback channel, making it possible to respond tochanging circumstances. In particular, in closed loop MIMO the UEprovides a RI; one or two CQI reports depending on the RI value; and aPrecoding Matrix Indicator (PMI).

Precoding is well known in the art and, in general, is applied to thedata carried on the Physical Downlink Shared Channel (PDSCH) in order toincrease the received Signal to Interference plus Noise power Ratio(SINR). This is done by setting different transmit antenna weights foreach transmission layer (stream) using channel information fed back fromthe UE. The ideal transmit antenna weights for precoding are generatedfrom eigenvector(s) of the covariance matrix of the channel matrix, H,given by HH^(H), where ^(H) denotes the Hermitian transpose. LTE Rel. 8uses codebook-based precoding, in which the best precoding weights amonga set of predetermined precoding matrix candidates (a codebook) isselected to maximize the total throughput on all layers after precoding,and the index of this matrix (the Precoding Matrix Indicator (PMI)) isfed back to the base station (eNode B).

FIG. 9A illustrates some basic aspects of conventional closed loop MIMOoperation. Act A-1 of FIG. 9A shows the wireless terminal sending one ormore report(s) of numerous parameters (including RI, the PrecodingMatrix Indicator (PMI), HARQ ACK/NAK, and CQIs for two codewords) to thenode. As act A-2 the base station node evaluates the wireless terminal'sreports. Act A-3 comprises the base station node making its decisionregarding the appropriate transmission mode. Then, as act A-4, the basestation node communicates its decision of the transmission mode to thewireless terminal using higher layer signaling. Upon receipt of thetransmission mode decided by the base station node as act A-5 thewireless terminal implements and acknowledges the transmission modedecided by the base station node. Until the time of suchacknowledgement, the base station operates with some ambiguity. Uponreceipt of the confirmation from the wireless terminal, as act A-6 thebase station node then begins to schedule transmissions to the wirelessterminal using the new transmission mode, e.g., the transmission modedecided at act A-3. Aspects of conventional open loop MIMO employ actssimilar to those shown in FIG. 9A, it being understood, however, that inopen loop MIMO for act A-1 the wireless terminal does not transmit thePMI and for each parameter transmits a value averaged over two codewordsrather than value for each of two codewords.

Simulation results appear to indicate that CL-MIMO has betterperformance than OL-MIMO when the mobile speed is low. However, when themobile speed is high, the advantage of CL-MIMO vanishes.

Given the fact that in CL-MIMO, the receiver reports channel status onan uplink control channel to the transmitter, a problem withconventional CL-MIMO in LTE is that the uplink control channel overheadincreases in direct proportion to the number of users. Specifically, LTECL-MIMO requires two channel quality (CQI) reports plus one PMI reportper user.

For a given UE the LTE system can switch between the CL-MIMO and OL-MIMOmodes. But the transition or switching may be slow. The transitionbetween modes requires Radio Resource Control (RRC) signaling which(typically) takes 100s of milliseconds. This reduces the ability of LTEto respond to rapidly changing radio frequency (RF) channel conditions,which in turn impacts system capacity.

SUMMARY

In one of its aspects the technology disclosed herein concerns a node ofa radio access network which communicates over a radio interface with awireless terminal comprises a transmitter and a controller. Thetransmitter selectively operates in plural transmission modes fordownlink transmission over the radio interface, at least one of theplural transmission modes being a multiple input multiple output (MIMO)transmission mode. The controller uses both a terminal speed value and athroughput value to make a determination when to switch between theplural transmission modes for communicating with the wireless terminal.

In an example embodiment the plural transmission modes include pluralMIMO modes. The plural MIMO modes comprise a first mode and a secondmode. In the first mode open loop MIMO operates with cyclical diversitydelay. In the second mode open loop MIMO operates without cyclicaldiversity delay. In basic operation, the controller is configured todetermine in which of the plural modes the node is to operate withrespect to the wireless terminal by performing basic act (1) throughbasic act (4). Basic act (1) and basic act (2) are performed when thenode is in the first mode; basic act (3) and basic act (4) are performedwhen the node is in the second mode.

Of the first mode acts, basic act (1), which depends on the terminalspeed value, comprises keeping the downlink transmission in the firstmode if the terminal speed value exceeds a terminal speed threshold.Basic act (2) is performed when the terminal speed value is below theterminal speed threshold, and comprises two sub-acts. Sub-act(2a) thecomprises switching the downlink transmission to the second mode when afirst mode throughput value exceeds a throughput threshold. Sub-act (2b)comprises keeping the downlink transmission in the first mode when thefirst mode throughput value does not exceed the throughput threshold.

Of the second mode acts, basic act (3), which also depends on theterminal speed value, comprises switching the downlink transmission tothe first mode if the terminal speed value exceeds the terminal speedthreshold. Basic act (4) is performed in the second mode when theterminal speed value does not exceed the terminal speed threshold, andcomprises two sub-acts. Sub-act(4a) comprises switching the downlinktransmission to the first mode when a second mode throughput value doesnot exceed the throughput threshold. Sub-act(4b) comprises keeping thedownlink transmission in the second mode when the second mode throughputvalue does exceed the throughput threshold.

In an example embodiment, in the first mode the controller is configuredto use a first precoding matrix indicator to determine a firstmode/first PMI throughput value which is used for the first modethroughput value when performing acts (1) and (2); and to use a secondprecoding matrix indicator to determine a first mode/second PMIthroughput value which is used for the first mode throughput value thenperforming acts (1) and (2). If the first mode/first PMI throughputvalue exceeds the first mode/second PMI throughput value, the controlleris configured to use the first mode/first PMI throughput value as thefirst mode throughput value when repeating acts (1) and (2). If thefirst mode/first PMI throughput value does not exceed the firstmode/second PMI throughput value, the controller is configured to thefirst mode/second PMI throughput value as the first mode throughputvalue when repeating acts (1) and (2). In an example implementationwherein feedback is obtained for both a first codeword and a secondcodeword transmitted on the downlink, the controller is furtherconfigured to determine at least one of the first mode/first PMIthroughput value and the first mode/second PMI throughput value by thesum of the first codeword throughput value and the second codewordthroughput value.

In an example embodiment, in the second mode the controller isconfigured to use a first precoding matrix indicator to determine asecond mode/first PMI throughput value which is used for the second modethroughput value when performing act (3) and to use a second precodingmatrix indicator to determine a second mode/second PMI throughput valuewhich is used for the second mode throughput value when performing act(3). The controller is configured to perform the following further actswhen in second first mode and when the terminal speed value does notexceeds the terminal speed threshold: compare a maximum of the secondmode/first PMI throughput value and the second mode/second PMIthroughput value to the throughput threshold; if the maximum of thesecond mode/first PMI throughput value and the second mode/second PMIthroughput value does not exceed the throughput threshold, switch thedownlink transmission to the first mode; but otherwise when the maximumof the second mode/first PMI throughput value and the second mode/secondPMI throughput value is the second mode/first PMI throughput value,repeat act (3) using the second mode/first PMI throughput value as thesecond mode throughput value, or when the maximum of the secondmode/first PMI throughput value and the second mode/second PMIthroughput value is the second mode/second PMI throughput value, repeatact (3) using the second mode/second PMI throughput value as the secondmode throughput value.

In an example embodiment and mode the controller is configured toestimate the throughput value by (1) using a channel quality indicationreported by the wireless terminal to determine a signal to interferenceplus noise ratio (SINR): (2) deriving a channel gain to interferencenoise ratio (GINR) from the SINR by subtracting a power spectrum densityof a reference signal (PSD_(RS)); (3) applying a smoothing filter to theGINR to obtain a smoothed GINR value; (4) using HARQ ACK/NACK feedbackto adjust the GINR to obtain an adjusted GINR; (5) using the smoothedGINR value, the adjusted GINR value, and a power spectrum density of aPDSCH signal to obtain the throughput value for each codeword. Thecontroller is configured to add the throughput value for a firstcodeword and a second codeword to obtain a final throughput value forthe multiple input multiple output (MIMO) mode.

In example embodiments and modes, various example techniques areprovided for the controller to determine the GINR adjusted value.

In an example embodiment and mode, the controller is configured to useHARQ feedback to adjust the throughput value.

In an example embodiment and mode, a transmitter of the base stationnode communicates the determination to the wireless terminal usingchannel condition-contemporaneous physical layer signaling.

In an example embodiment and mode, the plural transmission modes furthercomprise a third mode which is a rank one mode, and wherein the node isfurther configured to use a rank indicator value and/or the terminalspeed value to make the determination when to switch the downlinktransmission to or away from the rank one mode.

Example methods of operating a base station node and a communicationsnetwork are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of thetechnology disclosed herein will be apparent from the following moreparticular description of preferred embodiments as illustrated in theaccompanying drawings in which reference characters refer to the sameparts throughout the various views. The drawings are not necessarily toscale, emphasis instead being placed upon illustrating the principles ofthe technology disclosed herein.

FIG. 1A is a schematic view of a communications system including a nodewhich communicates over a radio interface with a wireless terminal andwhich switches between plural MIMO transmission modes.

FIG. 1B is a schematic view of a communications system including a nodewhich communicates over a radio interface with a wireless terminal andwhich switches between plural MIMO modes.

FIG. 2 is a schematic view of a communications system including a nodewhich communicates over a radio interface with a wireless terminal andwhich switches between three MIMO modes.

FIG. 3 is a flowchart showing basic acts performed by a MIMO mode switchcontroller according to an example embodiment.

FIG. 4 is a flowchart showing basic acts performed by a MIMO mode switchcontroller according in a first mode of open loop MIMO which operateswith cyclical diversity delay.

FIG. 5 is a flowchart showing basic acts performed by a MIMO mode switchcontroller according in a second mode of open loop MIMO which operateswithout cyclical diversity delay.

FIG. 6 is a flowchart showing basic acts performed by a MIMO mode switchcontroller according in a third mode (e.g., a rank one mode such as aspace frequency block code (SFBC) mode).

FIG. 7 is a flowchart showing basic acts performed by a MIMO mode switchcontroller shows in performing a throughput estimation technique.

FIG. 8 is a diagrammatic view illustrating switching between the threetransmission modes of FIG. 2.

FIG. 9A is a diagrammatic view depicting basic acts involved inselection of transmission mode in a conventional MIMO operation.

FIG. 9B is a diagrammatic view depicting basic acts involved inselection of transmission mode in MIMO operation according to an exampleembodiment of the technology disclosed herein.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth such as particulararchitectures, interfaces, techniques, etc. in order to provide athorough understanding of the technology disclosed herein. However, itwill be apparent to those skilled in the art that the technologydisclosed herein may be practiced in other embodiments that depart fromthese specific details. That is, those skilled in the art will be ableto devise various arrangements which, although not explicitly describedor shown herein, embody the principles of the technology disclosedherein and are included within its spirit and scope. In some instances,detailed descriptions of well-known devices, circuits, and methods areomitted so as not to obscure the description of the technology disclosedherein with unnecessary detail. All statements herein recitingprinciples, aspects, and embodiments of the technology disclosed herein,as well as specific examples thereof, are intended to encompass bothstructural and functional equivalents thereof. Additionally, it isintended that such equivalents include both currently known equivalentsas well as equivalents developed in the future, i.e., any elementsdeveloped that perform the same function, regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat block diagrams herein can represent conceptual views ofillustrative circuitry or other functional units embodying theprinciples of the technology. Similarly, it will be appreciated that anyflow charts, state transition diagrams, pseudocode, and the likerepresent various processes which may be substantially represented incomputer readable medium and so executed by a computer or processor,whether or not such computer or processor is explicitly shown.

The functions of the various elements including functional blocks,including but not limited to those labeled or described as “computer”,“processor” or “controller”, may be provided through the use of hardwaresuch as circuit hardware and/or hardware capable of executing softwarein the form of coded instructions stored on computer readable medium.Thus, such functions and illustrated functional blocks are to beunderstood as being either hardware-implemented and/orcomputer-implemented, and thus machine-implemented.

In terms of hardware implementation, the functional blocks may includeor encompass, without limitation, digital signal processor (DSP)hardware, reduced instruction set processor, hardware (e.g., digital oranalog) circuitry including but not limited to application specificintegrated circuit(s) [ASIC], and/or field programmable gate array(s)(FPGA(s)), and (where appropriate) state machines capable of performingsuch functions.

In terms of computer implementation, a computer is generally understoodto comprise one or more processors or one or more controllers, and theterms computer and processor and controller may be employedinterchangeably herein. When provided by a computer or processor orcontroller, the functions may be provided by a single dedicated computeror processor or controller, by a single shared computer or processor orcontroller, or by a plurality of individual computers or processors orcontrollers, some of which may be shared or distributed. Moreover, useof the term “processor” or “controller” shall also be construed to referto other hardware capable of performing such functions and/or executingsoftware, such as the example hardware recited above.

The technology can additionally be considered to be embodied entirelywithin any form of computer-readable memory, such as solid-state memory,magnetic disk, or optical disk containing an appropriate set of computerinstructions that would cause a processor to carry out the techniquesdescribed herein.

FIG. 1A shows an example radio communications network 20 comprising anode 28 and at least one wireless terminal 30. The node 28 communicateswith at least one, and typically many, wireless terminals over radiointerface 32. In basic construction pertinent to the present technology,node 28 comprises transmitter 34 and controller 36. The transmitter 34selectively operates in plural transmission modes for downlinktransmission over radio interface 32 to wireless terminal 30. At leastone of the plural transmission modes is a mode with rank 1 (R1)transmission capability; at least another of the plural transmissionmodes is a mode with transmission capability greater than rank 1, e.g.,a multiple input multiple output (MIMO) mode. As such, transmitter 34comprises plural antennae 38. The antennae 38 are fed respective signalswhich have been processed for transmission.

The node 28 may be any type of radio access node which transmits overradio interface 32 to a wireless terminal. For example, node 28 may be abase station node, either macro or micro (such as a femto or pico basestation, for example). The node 28 may be either a donor node or a relaynode. The node 28 is not confined to any particular radio access networktechnology, although in the illustrated embodiments it has particularlybeneficial employment in Long Term Evolution (LTE). Moreover, it will beappreciated that node 28 typically has other units or elements notillustrated in FIG. 1B, such as (for example) interfaces to other nodes(e.g., other base stations) and interfaces to other networks, such asone or more core networks.

The controller 36 may control or supervise some or all of theoperational aspects of node 28, including such operational aspects asestablishing one or more bearers for communication between node 28 andwireless terminal 30; scheduling of frames for transmission on adownlink (DL) from node 28 to wireless terminal 30 and/or frames fortransmission on an uplink (UL) from wireless terminal 30 to node 28;formatting such frames for transmission on the downlink (DL) andprocessing frames received on the uplink (UL); power control for one orboth of node 28 and wireless terminal 30; diversity combining; andhandover, just to name a few. For the technology disclosed herein, thecontroller 36 includes control logic for making a determination when toswitch between the plural transmission modes. For convenience, suchcontrol logic and the apparatus which store or host the control logicare also referred to as transmit switch controller 40.

The transmission modes of the example embodiment of FIG. 1 may be any ofthe transmission modes of Table 1. However, at least one of thetransmission modes preferably has rank 1 (R1) transmission capability,e.g., a rank one transmission sub-mode, and at least another of thetransmission modes has transmission capability greater than rank one,e.g., is a multiple input multiple output (MIMO) transmission mode. Asan example, a transmission mode with rank 2 capability is shown in FIG.1A.

Transmission modes particularly suitable for the technology disclosedherein are transmission modes 4, 7, and 8 of Table 1, although othertransmission modes may also be employed.

FIG. 1B shows a specific case of the example embodiment of FIG. 1A,again showing an example radio communications network 20 comprising anode 28 and at least one wireless terminal 30. In basic constructionnode 28 of FIG. 1B also comprises transmitter 34 and controller 36. Thetransmitter 34 of FIG. 1B selectively operates in plural multiple inputmultiple output (MIMO) modes for downlink transmission over radiointerface 32 to wireless terminal 30. As such, transmitter 34 alsocomprises plural antennae 38. The antennae 38 are fed respective signalswhich have been processed for transmission according to, e.g., MIMOtechniques. Since the dual antenna configuration is the most widely usedin LTE networks, the examples of the present disclosure focus on thedual antenna scenario. However, the basic techniques disclosed hereinmay also be applied to other antenna configurations.

As understood by the person skilled in the art, 2×2 MIMO transmissiongenerally involves mapping of modulated symbols to two spatial layersfrom two codewords; precoding (multiplying the signal using a precodingmatrix with size 2×2); mapping both sets of precoded values ontoresource elements of a frame; generating OFDM signals to express theresource elements; and, applying the OFDM signals to two antenna portsof the MIMO antenna array. In an example embodiment, the foregoing MIMOfunctions may be performed by controller 36 of node 28.

In a basic example embodiment, the plural MIMO modes in whichtransmitter 34 selectively operates includes a first mode and a secondmode. The first mode is open loop MIMO which operates with cyclicaldiversity delay. The second mode is open loop MIMO which operateswithout cyclical diversity delay. One or both of the first mode and thesecond mode may have rank 1 (R1) capability. As is known in the art,Cyclic Delay Diversity (CDD) is a diversity scheme used in OFDM-basedtelecommunication systems, transforming spatial diversity into frequencydiversity avoiding inter-symbol interference (ISI). In essence, CDDshifts the transmit signal in the time direction and transmits themodified (e.g., shifted) signal copies over separate transmit antennas.The transmit-antenna specific signal modifications, i.e., the timeshifts, are inserted cyclically, such that no additional inter symbolinterference (ISI) occurs.

In terms of the switch of MIMO modes, for a first example embodimentFIG. 1B shows both the first mode (open loop MIMO which operates withcyclical diversity delay) and the second mode (open loop MIMO whichoperates without cyclical diversity delay) and further illustrates byarrows switches between the modes. For example, arrow M1-M2 illustratesa switch from the first mode to the second mode, and arrow M2-M1illustrates a switch from the second mode to the first mode. Accordingto the technology disclosed herein, in general the first mode (open loopMIMO which operates with cyclical diversity delay) is used when there isa sufficiently high terminal speed, which indicates that the speed ofthe wireless terminal 30 is high and the channel is changing quickly.Under such (high Doppler) conditions, there is no benefit to change PMI.

FIG. 2 shows, for another example embodiment, that the plural MIMO modesmay also include other modes, such as a third mode. In the example ofFIG. 2B the third mode happens to be a rank one mode (RI=1), such as aspace frequency block code (SFBC) mode, although other rank one (R1)modes could be involved in the switching as well (such as beamforming ortransmission mode 4, rank 1, for example). As such, FIG. 2 further showsby arrow M1-M3 a switch from the first mode to the third mode; by arrowM2-M3 a switch from the second mode to the third mode; by arrow M3-M1 aswitch from the third mode to the second mode; and, by arrow M3-M2 aswitch from the third mode to the second mode. Thus, the second exampleembodiment thus involves transmission mode 3 of OL-MIMO (see Table 1)with large CDD and SFBC and transmission mode 4 of CL-MIMO in this work.In the technology disclosed herein, the closed loop MIMO is replaced bythe OL-MIMO without CDD (in which PMI is not required to be sent fromUE). Using the technology disclosed herein, the best transmission schemeis selected adaptively to achieve optimum performance.

It should be appreciated that more than three modes may be utilized. Forexample, in another example embodiment the three modes of FIG. 2 may beutilized, and other modes as well. Such other modes may include, forexample, transmission mode 4 (Rank 1), transmission mode 7, andtransmission mode 8 from Table 1.

As described herein in more detail, transmit switch controller 40 usesboth a throughput value (e.g., TPV 41) and a terminal speed value(terminal speed value [TSV] 42) and to make a determination when toswitch between the plural transmit modes, for communicating with thewireless terminal 30. As used herein, “throughput” or “throughput value”is a number of information bits per resource element (RE). Thethroughput value may be different, e.g., differently determined, fordifferent MIMO modes.

In an example implementation, the terminal speed value may be a Dopplershift value obtained with reference to the wireless terminal. It can bederived by comparing the phase changes between two reference symbols. Asan alternative to a Doppler measurement the Transmit switch controller40 may also use the filtered time adjustment to estimate the UE speed.For example, the rate of delay change (distance) can be used to estimatethe terminal speed (Doppler).

FIG. 1B also shows some non-exhaustive details of wireless terminal 30,including a MIMO-compatible transceiver 44 and wireless terminalcontroller 46. The transceiver 44 comprises an appropriate antennastructure 48 as well as signal processing circuitry for signals receivedfrom node 28 on the downlink (DL) and circuitry for preparing signalsfor transmission to node 28 on the uplink (UL). The controller 36processes data received (e.g., in frames) on the downlink (DL) andprepares frames for transmission on the uplink (UL); cooperates in apower control loop; and provides various reports and information to node28, including the channel quality indicator or index (CQI) upon which achannel gain to interference noise ratio (GINR) is based.

In basic operation, transmit switch controller 40 is configured todetermine in which of the plural transmit modes the node is to operatewith respect to downlink transmissions to the wireless terminal byperforming basic act (1) through basic act (4) of FIG. 3. Basic act (1)and basic act (2) are performed when the node is in the first mode;basic act (3) and basic act (4) are performed when the node is in thesecond mode.

Of the first mode acts, basic act (1) of FIG. 3, comprises checking theterminal speed value (TSV), and then keeping the downlink transmissionin the first mode if the downlink (DL) transmission is at that time inthe first mode and the terminal speed value exceeds a terminal speedthreshold ((TSV>T1). Basic act (2) is performed when the terminal speedvalue does not exceed the terminal speed threshold (TSV<T1), compriseschecking a first mode throughput value (TPV₁), and further comprises twosub-acts. Sub-act (2a) comprises switching the downlink transmission tothe second mode when the first mode throughput value (TPV₁) exceeds athroughput threshold (T2). Sub-act (2b) comprises keeping the downlinktransmission in the first mode when the first mode throughput value(TPV₁) is less than the throughput threshold (T2).

Of the second mode acts, basic act (3), which also depends on andinvolves checking the terminal speed value, comprises switching thedownlink transmission to the first mode if the terminal speed valueexceeds the terminal speed threshold (TSV>T1). Basic act (4) isperformed in the second mode when the terminal speed value does notexceed the terminal speed threshold (TSV<T1), and comprises twosub-acts. Sub-act (4a) comprises switching the downlink transmission tothe first mode when a second mode throughput value (TPV₂) does notexceed the throughput threshold (T2). Sub-act (4b) comprises keeping thedownlink transmission in the second mode when the second mode throughputvalue (TPV₂) does exceed the throughput threshold (T2).

Basic example acts or steps of the first mode (open loop MIMO whichoperates with cyclical diversity delay) and second mode (open loop MIMOwhich operates without cyclical diversity delay) as executed by transmitswitch controller 40 are shown in FIG. 4 and FIG. 5, respectively. Amongthe acts involved, FIG. 4 shows the conditions needed to switch fromOL-MIMO with large CDD to SFBC or OL-MIMO without CDD. Conversely, FIG.5 shows the conditions needed to switch from OL-MIMO without CDD to SFBCor OL-MIMO with large CDD. FIG. 5 shows the conditions needed to switchfrom SFBC to OL-MIMO with large CDD or from SFBC to OL-MIMO without CDD.

FIG. 4 illustrates two alternative entry points for the first mode (openloop MIMO which operates with cyclical diversity delay): entry point4-0(A) and entry point 4-0(B). Similarly, FIG. 5 illustrates twoalternative entry points for the second mode (open loop MIMO whichoperates without cyclical diversity delay): entry point 5-0(C) and entrypoint 5-0(D). FIG. 5 illustrates only one entry point for the third mode(space frequency block code [SFBC] mode): entry point 6-0(E). For sakeof brevity, these five entry points are also shown in FIG. 4, FIG. 5,and FIG. 6 as symbols which internally include an appropriate one of theletters “A”-“E”, respectively, and textual reference to these entrypoints may also simply be in terms of the letters “A”-“E”.

As indicated above, Rank Indication (RI) is reported by wirelessterminal 30. The Rank Indication (RI) determines whether the transmitswitch controller 40 switches to space frequency block code (SFBC) ornot, e.g., whether the transmit switch controller 40 jumps to entrypoint 6-0(E) of FIG. 6. As a general rule, when RI=1, SFBC is used.Alternatively, when RI=2, either first mode (open loop MIMO whichoperates with cyclical diversity delay) or second mode (open loop MIMOwhich operates without cyclical diversity delay) is employed.

FIG. 4 comprises essentially two branches of logic: a first branch or“A” branch which may be initiated through entry point 4-0(A) and asecond branch or “B” branch which may be initiated through entry point4-0(B). Similarly, FIG. 5 comprises essentially two branches of logic: afirst branch or “C” branch which may be initiated through entry point4-0(C) and a second branch or “D” branch which may be initiated throughentry point 4-0(D). The two branches of FIG. 4 are essentially mirrorimages of each other, although some of the acts of one branch may usedifferent variables (and consequently different variable values) thanits mirrored branch. The same quasi-mirroring also exists with branchesC and D of FIG. 5.

Differences between the two branches include a different PrecodingMatrix Indicator (PMI) used for each, which in turn affects the SINR(Signal to Interference plus Noise Ratio) value and the throughput value(which are ultimately determined by which Precoding Matrix Indicator(PMI) is employed for the branch).

For the case of Rank Indication (RI)=2, two PMIs are considered in boththe first mode (open loop MIMO which operates with cyclical diversitydelay) and the second mode (open loop MIMO which operates withoutcyclical diversity delay). The two PMIs are described in Expression 1.

$\begin{matrix}{{{P\; M\; I_{1}} = {\frac{1}{2}\begin{pmatrix}1 & 1 \\1 & {- 1}\end{pmatrix}}}{and}{{P\; M\; I_{2}} = {\frac{1}{2}\begin{pmatrix}1 & 1 \\j & {- j}\end{pmatrix}}}} & {{Expression}\mspace{14mu} (1)}\end{matrix}$

In general, the left branch of FIG. 4 (the “A” branch) and the leftbranch of FIG. 5 (the “C” branch) are processed based on the firstPrecoding Matrix Indicator (PMI₁), and the right branch of FIG. 4 (the“B” branch) and the right branch of FIG. 5 (the “D” branch) areprocessed based on the second Precoding Matrix Indicator (PMI₂).

Besides Rank Indication (RI) and terminal speed, the throughput value isalso used as input to the determination whether to switch betweendifferent PMIs. In FIG. 4 and FIG. 5 the throughput value is denoted by“TPV”, and is determined by the signal to noise and interference ratio(SINR) in a manner such as that described herein by way of example.

As mentioned previously, the throughput, and thus the throughput value,is the number of information bits per resource element (RE). As usedherein, in some instances the throughput value is generally representedas TVP, in other instances the throughput value is exemplified as havingone subscript (e.g., TVP_(i)), and in yet other instances the throughputvalue is exemplified as having two subscripts (e.g., TVP_(ij)). In suchnotation, the first subscript (“i”) indicates the PMI number and thesecond subscript (“j”) indicates a code word. For instance, TPV₁₀represents the throughput value for PMI₁ and code word 0; TPV₂₁represents the throughput value for PMI₂ and code word 1; etc. Ingeneral, within a mode the PMI with the higher throughput value will beselected.

A counter “M” is used in the techniques depicted by FIG. 4 and FIG. 5 toindicate if a throughput value for a different PMI is available or not.The above-mentioned comparison of throughput values can only be donewhen the throughput values for both PMIs are available.

In executing the logic of FIG. 4 and FIG. 5 for the first mode (openloop MIMO which operates with cyclical diversity delay) and second mode(open loop MIMO which operates without cyclical diversity delay),respectively, the transmit switch controller 40 uses a throughputthreshold (T2) in making its determination whether the OL-MIMO withlarge CDD or OL-MIMO without CDD should be used. In general, when thethroughput value is low, the OL-MIMO with large CDD should be used.Fortuitously, in the first mode (open loop MIMO which operates withcyclical diversity delay), the throughput value for each code word isthe same, and thus some simplifications of the implementation can berealized. But the second mode (open loop MIMO which operates withoutcyclical diversity delay) involves two codewords, e.g., a first codewordand a second codeword, and the throughput values are different withrespect to those two codewords.

The two branches of FIG. 4, e.g., branch A of FIG. 4 and branch B ofFIG. 4 generally follow a similar logic scheme. Therefore, branch A andbranch B are herein generally discussed collectively. In general, thenomenclature X—Y(Z) is employed to discuss the respective acts of FIG. 4and FIG. 5, with X representing the figure and the preamble of each act(e.g., FIG. 4 or FIG. 5), Y representing the number of the act in thebranch (e.g., 0 through 8); and Z representing the branch (e.g., branchA, B, C, or D).

After entry at the respective entry point act 4-0(Z), at act 4-1(Z) eachof branch A and branch B initializes its counter M to zero. It will berecalled that “M” is used in FIG. 4 to indicate if a throughput valuefor a different PMI is available or not.

As mentioned above, a first Precoding Matrix Indicator (PMI₁) isapplicable to both branch A of FIG. 4 and branch C of FIG. 5 and asecond Precoding Matrix Indicator (PMI₂) is applicable to both branch Bof FIG. 4 and branch D of FIG. 5. In conventional practice PMI may becalculated by the wireless terminal based on the channel conditionderived from the received signals. However, in the technology disclosedherein the node 28 (e.g., eNB) decides which PMI will be used based onthroughput (e.g., the throughput value). Moreover, the PMIs may, atleast in some example embodiments, be fixed. For the two actsgenerically represented as act 4-2(Z), Table 2 illustrates the valuesare determined for each act:

TABLE 2 VALUES DETERMINED FROM PMI Act 4-2(A) Act 4-2(B) Act 5-2(C) Act5-2(D) SINR₁₀ -> TPV₁₀ SINR₂₀ -> TPV₂₀ SINR₁₀ -> TPV₁₀ SINR₂₀ -> TPV₂₀SINR₁₁ -> TP₁₁ SINR₂₁ -> TPV₂₁ SINR₁₁ -> TPV₁₁ SINR₂₁ -> TPV₂₁ TPV₁ =TPV₁₀ + TPV₁₁ TPV₂ = TPV₂₀ + TPV₂₁ TPV₁ = TPV₁₀ + TPV₁₁ TPV₂ = TPV₂₀ +TPV₂₁

Act 4-3(Z) comprises the transmit switch controller 40 checking todetermine whether the Rank Indication (RI) is equal to 2. If the RankIndication (RI) is less than 2, the transmit switch controller 40 jumpsto the space frequency block code (SFBC) mode, e.g., entry point “E” oract 6-0(E) of FIG. 6. If the Rank Indication (RI) does equal 2, a checkis made by transmit switch controller 40 as act 4-4(Z) whether theappropriate throughput value for the respective branch Z is greater thana second threshold, the throughput threshold (T2). Like the thresholdT1, the throughput threshold T2 may also be derived from simulationand/or field measurements. The thresholds T1 and T2 may be the same forall branches.

In the first mode, if the check of act 4-4(Z) is positive, e.g., if thethroughput value is greater than the second threshold, as act 4-5(Z) thetransmit switch controller 40 checks whether the terminal speed valueexceeds a first threshold, e.g., a terminal speed threshold T1. Theterminal speed threshold T1 may be derived from simulation and/or fieldmeasurements. If the terminal speed value TSV does not exceed the firstthreshold T1, the transmit switch controller 40 knows to jump to thesecond mode (open loop MIMO which operates without cyclical diversitydelay), e.g., to jump either to act 5-0(C) or to act 5-0(D). Whether thejump occurs to act 5-0(C) or to act 5-0(D) may be randomly determined,and it is not important at any particular time whether the jump is toact 5-0(C) or to act 5-0(D). It is preferred, however, that when jumpingto another mode that jumps to different entry points be approximatelyequally distributed, or at least that the jump not always orpredominately be to any particular one of the two possible entry pointsC or D. The same is true for other jumps described herein, e.g., a jumpfrom the second mode to act 4-0(A) or to act 4-0(B).

In the first mode (open loop MIMO which operates with cyclical diversitydelay), if the terminal speed value exceeds the terminal speed thresholdT1, or if the throughput value for the respective branch Z is less thanthe second threshold T2, the transmit switch controller 40 continueswith act 4-6(Z).

As act 4-6(Z) the transmit switch controller 40 checks whether the valueof counter M is zero. If the counter M is zero, the transmit switchcontroller 40 realizes that it has not yet been able to compute athroughput value for both Precoding Matrix Indicators (PMIs), and thenprepares to do so by executing act 4-7(Z). Act 4-7(Z) comprisesincrementing counter M (e.g., M=M+1). Then transmit switch controller 40jumps to act 4-2(Z) of the mirrored branch. For example, after act4-7(A) transmit switch controller 40 jumps to act 4-2(B) in order todetermine the values which appear in the second column of Table 2.Conversely, after act 4-7(B) the transmit switch controller 40 jumps toact 4-2(A) in order to determine the values which appear in the firstcolumn of Table 2. Executing the act 4-2(Z) enables the transmit switchcontroller 40 to obtain both throughput values for the mode.

If the counter M as determined at act 4-6(Z) is not zero, transmitswitch controller 40 executes act 4-8(Z) of each branch in order tocompare the two values TPV₁ and TPV₂. Whichever of the two values TPV₁and TPV₂ is the greater determines which in branch of the mode thetransmit switch controller 40 will continue to execute. For example, inthe first mode, if TPV₁ is greater than TPV₂, execution will remain inor jump to branch A; if TPV₁ is not greater than TPV₂, execution willremain in or jump to branch B.

Thus, it can be seen that for the first mode (open loop MIMO whichoperates with cyclical diversity delay) of FIG. 4 the transmit switchcontroller 40 uses a first precoding matrix indicator to determine afirst mode/first PMI throughput value which is used for the first modethroughput value when performing act 4-4(A), but uses a second precodingmatrix indicator to determine a first mode/second PMI throughput valuewhich is used for the first mode throughput value when performing act4-4(B). Further, if the first mode/first PMI throughput value exceedsthe first mode/second PMI throughput value (as determined at either ofacts 4-8(A) or 4-8(B), the transmit switch controller 40 uses the firstmode/first PMI throughput value as the first mode throughput value whenrepeating act 4-4(A). On the other hand, if the first mode/first PMIthroughput value does not exceed the first mode/second PMI throughputvalue, the transmit switch controller 40 uses the first mode/second PMIthroughput value as the first mode throughput value when repeating act4-4(B).

The two branches of and FIG. 5, e.g., branch C of FIG. 5, and branch Dof FIG. 5, generally follow a similar logic scheme. Therefore, these twobranches are herein generally discussed collectively, with thenomenclature X—Y(Z) being employed to discuss the respective acts, inwhich X represents the figure and the preamble of each act (i.e., FIG.5), Y represents the number of the act in the branch (e.g., 0 through8); and Z represents the branch (e.g., branch C, or D).

After entry at the respective entry point act 5-0(Z), at act 5-1(Z) eachbranch initializes its counter M to zero. It will be recalled that “M”is used to indicate if a throughput value for a different PMI isavailable or not.

As understood from the foregoing, a first Precoding Matrix Indicator(PMI₁) is applicable to branch C of FIG. 5 and a second Precoding MatrixIndicator (PMI₂) is applicable to branch D of FIG. 5. For the two actsgenerically represented as act 5-2(Z), Table 2 above illustrates thevalues are determined for each act.

Act 5-3(Z) comprises the transmit switch controller 40 checking todetermine whether the Rank Indication (RI) is equal to 2. If the RankIndication (RI) is not equal to 2, the transmit switch controller 40jumps to the space frequency block code (SFBC) mode, e.g., entry point“E” or act 6-0(E) of FIG. 6. If the Rank Indication (RI) is equal to 2,a check is made by transmit switch controller 40 as act 5-4(Z) whetherthe terminal speed value exceeds the first threshold, e.g., the terminalspeed threshold T1. As indicated above, in the second mode (open loopMIMO which operates without cyclical diversity delay), if the terminalspeed value TSV exceeds the terminal speed threshold T1, transmit switchcontroller 40 switches from the second mode to the first mode, e.g.,jumps either to act 4-0(A) or to act 4-0(B). As mentioned above, whetherthe jump occurs to act 4-0(A) or to act 4-0(B) may be randomlydetermined. If it is determined at act 5-4(Z) that the terminal speedvalue does not exceed the first threshold, e.g., the terminal speedthreshold T1, as act 5-5(Z) the transmit switch controller 40 checks ifthe value of counter M is zero. If the counter M is zero, the transmitswitch controller 40 realizes that it has not yet been able to compute athroughput value for both Precoding Matrix Indicators (PMIs), and thenprepares to do so by executing act 5-6(Z). Act 5-6(Z) comprisesincrementing counter M (e.g., M=M+1). After incrementing the counter Mthe transmit switch controller 40 jumps to act 5-2(Z) of the mirroredbranch. For example, after act 5-7(C) transmit switch controller 40jumps to act 5-2(D) in order to determine the values which appear in thefourth column of Table 2. Conversely, after act 5-7(D) the transmitswitch controller 40 jumps to act 5-2(C) in order to determine thevalues which appear in the third column of Table 2. Executing the act5-2(Z) enables the transmit switch controller 40 to obtain boththroughput values for the mode.

If it is determined at act 5-5(Z) that the counter M is not zero, as act5-7(Z) the transmit switch controller 40 checks whether the throughputvalue for branch C (e.g., TPV₁) is greater than the throughput value forbranch D (e.g., TPV₂). If it is determined as act 5-7(C) that TPV₁ isnot greater than TPV₂, execution jumps to act 5-8(D). But if it isdetermined as act 5-7(C) that TPV₁ is greater than TPV₂, then executioncontinues at act 5-8(C). Conversely, if it is determined as act 5-7(D)that TPV₁ is greater than TPV₂, execution jumps to act 5-8(C). But if itis determined as act 5-7(D) that TPV₁ is not greater than TPV₂, thenexecution continues at act 5-8(D).

Act 5-8(Z) involves the transmit switch controller 40 checking whetherthe appropriate throughput value for the respective branch Z is greaterthan the second threshold, the throughput threshold (T2). Like thethreshold T1, the throughput threshold T2 may also be derived fromsimulation and/or field measurements. The thresholds T1 and T2 may bethe same for all branches. If the throughput value for the respectivebranch Z is greater than the second threshold, the throughput threshold(T2), execution remains in the same branch (e.g., execution jumps backto act 5-2(Z)). But if the throughput value for the respective branch Zis not greater than the second threshold, the transmit switch controller40 knows to jump to the first mode (open loop MIMO which operates withcyclical diversity delay), e.g., to jump either to act 4-0(A) or to act4-0(B).

Thus it can be seen that when performing the second mode (open loop MIMOwhich operates without cyclical diversity delay) the transmit switchcontroller 40 uses a first precoding matrix indicator PMI₁ to determinea second mode/first PMI throughput value which is used for the secondmode throughput value when performing branch C including act 5-7(C), anduses a second precoding matrix indicator PMI₂ to determine a secondmode/second PMI throughput value which is used for the second modethroughput value when performing branch D including act 5-7(D). In thesecond mode the transmit switch controller 40 compares a maximum of thesecond mode/first PMI throughput value (TPV₁) and the second mode/secondPMI throughput value (TPV₂) [such maximum being determined at act 5-7(c)and 5-7(D)] to the throughput threshold (T2). If the maximum of thesecond mode/first PMI throughput value and the second mode/second PMIthroughput value does not exceed the throughput threshold, the transmitswitch controller 40 switches the downlink transmission to the firstmode. Otherwise, (a) if it is determined at act 5-8(C) that the maximumof the second mode/first PMI throughput value and the second mode/secondPMI throughput value is the second mode/first PMI throughput value,branch C of the second mode is repeated using the second mode/first PMIthroughput value as the second mode throughput value; or (b) If it isdetermined at act 5-8(D) that the maximum of the second mode/first PMIthroughput value and the second mode/second PMI throughput value is thesecond mode/second PMI throughput value, branch D of the second mode isrepeated using the second mode/second PMI throughput value as the secondmode throughput value.

FIG. 6 illustrates example acts or steps which are performed in a thirdmode (space frequency block code (SFBC) mode). Act 6-1 generallyreflects operation of the base station node and wireless terminal in thespace frequency block code (SFBC) mode. Periodically or as otherwiseappropriate as act 6-2 the transmit switch controller 40 checks toensure that the Rank Indicator (RI) is equal to 1, e.g., to ensure thatthe space frequency block code (SFBC) mode is still appropriate. If theRank Indicator (RI) is equal to 1, execution remains in the spacefrequency block code (SFBC) mode. But if the Rank Indicator (RI) haschanged to a value other than 1, act 6-3 is performed. Act 6-3 compriseschecking if the terminal speed value (TSV) is greater than the terminalspeed threshold (T1). If the terminal speed value (TSV) is greater thanthe terminal speed threshold (T1), execution jumps to the first mode(open loop MIMO which operates with cyclical diversity delay), e.g., toact 4-0(A) or act 4-0(B) (see FIG. 4). On the other hand, if theterminal speed value (TSV) is not greater than the terminal speedthreshold (T1), execution jumps to the second mode (open loop MIMO whichoperates without cyclical diversity delay), e.g., to act 5-0(C) or act5-0(D) (see FIG. 5). When jumping to any particular mode, the jump mayrandomly occur to either branch of the mode. The node 28 estimatesthroughput value (TPV). First, the Signal to Interference plus Noisepower Ratio (SINR) is estimated based on the channel quality indicator(CQI) report. Then the throughput value is derived from SINR through alook-up table. To combat systematic errors between the wireless terminalmeasurement and the base station link adaptation, an outer-loopadjustment of the gain to interference noise ratio (GINR) based on HARQACK/NACK feedback is used. For OL-MIMO without CDD, the base stationtracks the GINR (channel Gain to Interference plus Noise Ratio) for eachcodeword.

FIG. 7 shows basic acts or steps comprising a throughput estimationtechnique according to an example, non-limiting implementation. Act 7-1comprises the node 28 receiving the CQI report from the wirelessterminal 30. Act 7-2 comprises mapping the CQI to SINR using a tablederived from receiver link models. Act 7-3 comprises deriving GINR fromSINR by subtracting the power spectrum density of the reference signal(PSD_(RS)). Act 7-4 comprises applying a smoothing filter to GINR tofollow the slow fading. Act 7-5 shows receipt of HARQ ACK/NACK feedback.Act 7-6 comprises using the HARQ ACK/NACK feedback to adjust GINR tocombat systematic errors between UE measurement and node linkadaptation. Act 7-7 comprises the SINR calculation by adding thefilter-smoothed GINR value, the effect of the HARQ ACK/NACK feedback,and the power spectrum density of PDSCH signal (PSD_(PDSCH)). Act 7-8depicts the output of the resultant throughput value estimation, whichcan be used for both transmission switching and link adaptationalgorithm performed by controller 36. Link adaptation, or adaptivecoding and modulation (ACM), is employed by controller 36 for matchingof the modulation, coding and other signal and protocol parameters tothe conditions on the radio link (e.g., the pathloss, the interferencedue to signals coming from other transmitters, the sensitivity of thereceiver, the available transmitter power margin, etc.). It will beunderstood from the foregoing that HARQ feedback is used to adjust theGINR, and thus as a consequence to adjust the throughput value.

For the first mode (open loop MIMO which operates with cyclicaldiversity delay), throughput value estimation for one codeword isrequired since both code words experience the same channel condition.For the second mode (open loop MIMO which operates without cyclicaldiversity delay), on the other hand, the throughput value needs to beestimated separately for each code word, since the two code words havedifferent channel conditions.

By way of example, FIG. 8 illustrates switching between the threetransmission modes of FIG. 2, and the criteria or parameters that may beutilized by transmit switch controller 40 in conjunction with theswitching decision. The switching between the first mode (open loop MIMOwhich operates with cyclical diversity delay) and the second mode (openloop MIMO which operates without cyclical diversity delay) may be basedon the terminal speed value (TSV) [e.g., Doppler] and the throughputvalue (TPV), as discussed above. The switching between the third mode(SFBC mode) and either of the first mode (open loop MIMO which operateswith cyclical diversity delay) and the second mode (open loop MIMO whichoperates without cyclical diversity delay) may be based on the rankindicator (RI) and the throughput value (TPV). When the rank indicator(RI) has a value of “1”, the transmit switch controller 40 must switchto the third mode (SFBC mode). But a rank indicator (RI) of greater than“1” does not necessarily mean that a rank one transmission mode ofgreater than “1” must be selected. While it is true that a higher therank indicator generally suggests switching to a MIMO transmission mode,in some instances the channel quality may be too low to justify a MIMOtransmission mode. In other words, if the throughput value (TPV) is toolow, a rank one transmission mode such as SFBC may be selected even ifthe rank indicator (RI) is greater than one.

To facilitate the ensuing discussion of how the GINR (channel Gain toInterference plus Noise Ratio) values are derived or adjusted, Table 3provides terminology explanations.

TABLE 3 GINR TERMINOLOGY Terminology Explanation GINR_init The initialGINR derived from the filtered CQI reports GINR_Adj The GINR adjustmentdetermined from HARQ feedbacks GINR The final GINR determined from thesum of GINR and GINR_Adj GINR_UpStep GINR step size for upgrade whichdepends on the BLER target GINR_DownStep GINR step size for downgradewhich depends on the BLER target

With the benefit of FIG. 7 and the above explanations, varioustechniques for GINR outer-loop adjustment are now described, including(in presentation order) GINR outer-loop adjustment for space frequencyblock code (SFBC) [mode 3]; GINR outer-loop adjustment for the firstmode (open loop MIMO which operates with cyclical diversity delay); andGINR outer-loop adjustment for the second mode (open loop MIMO whichoperates without cyclical diversity delay).

Since space frequency block code (SFBC) [third mode] has only onecodeword, its GINR outer-loop adjustment is understood with reference toExpressions (2). GINR is derived from SINR and PDSCH signal powerspectrum density as shown in FIG. 7.

GINR_Adj=GINR_Adj+GINR_UpStep if HARQ is ACK

GINR_Adj=GINR_Adj−GINR_DownStep if HARQ is NACK  Expressions (2)

The final GINR may be calculated as shown in Expression (3).

GINR=GINR_init+GINR_Adj  Expression (3)

For the first mode (open loop MIMO which operates with cyclicaldiversity delay), GINR estimation for one codeword is required sinceboth code words experience the same channel condition. If HARQ for bothcode words is ACK, Expression (4) is employed. If HARQ for both codewords is NACK, Expression (5) is employed. If HARQ for one code word isACK and the other is NACK, Expression (6) is employed. The final GINRfor both code words can be calculated as Expression (7).

GINR_Adj=GINR_Adj+2*GINR_UpStep  Expression (4)

GINR_Adj=GINR_Adj−2*GINR_DownStep  Expression (5)

GINR_Adj=GINR_Adj+GINR_UpStep−GINR_DownStep  Expression (6)

GINR=GINR_init+GINR_Adj  Expression (7)

From the foregoing it can thus be understood that, in an exampleimplementation of the first mode, feedback is obtained for both a firstcodeword and a second codeword transmitted on the downlink. In suchexample implementation the controller is configured to determine atleast one of the first mode/first PMI throughput value and the firstmode/second PMI throughput value using an initial value and adifferential value. In an example embodiment, the first mode throughputvalue (whether for the first PMI or the second PMI) is determined by:adding twice a ratio up-step value to a previous ratio value if thefeedback for both the first codeword and the second codeword is apositive acknowledgement; subtracting twice a ratio down-step value fromthe previous ratio value if the feedback for both the first codeword andthe second codeword is a negative acknowledgement; and, adding adifference between the ratio up-step value and the ratio down-step valueto the previous ratio value if the feedback for the one codeword is apositive acknowledgement and the feedback for the other codeword is anegative acknowledgement.

For the second mode (open loop MIMO which operates without cyclicaldiversity delay), the GINR needs to be estimated separately for eachcode word, since they experience different channel conditions.Accordingly, the GINR estimation for the second mode results in twoestimations: GINR0 and GINR1, as reflected by Expression (11) andExpression (12), respectively. Preparatory acts leading to the eventualestimates involve estimating GINR_Adj0 and GINR_Adj1. GINR_Adj0 may beestimated using an appropriate one of Expression (7) and Expression (8).GINR_Adj1 may be estimated using an appropriate one of Expression (9)and Expression (10).

GINR_Adj0=GINR_Adj0+GINR_UpStep if HARQ for codeword 0 isACK  Expression (7)

GINR_Adj0=GINR_Adj0−GINR_DownStep if HARQ for codeword 0 isNACK  Expression (8)

GINR_Adj1=GINR_Adj1+GINR_UpStep if HARQ for codeword 1 isACK  Expression (9)

GINR_Adj1=GINR_Adj1−GINR_DownStep if HARQ for codeword 1 isNACK  Expression (10)

GINR0=GINR_init+GINR_Adj0  Expression (11)

GINR1=GINR_init+GINR_Adj1  Expression (12)

At the very beginning of operation, when the wireless terminal isconnected to the system, the most conservative GINR_Adj=GINR_Adj_Int isused. Thereafter, the GINR_Adj value can be re-used when thetransmission mode is switched from one mode to another. For example, ifthe transmission mode is switched from either SFBC or the first mode tothe second mode, the initial value for two code words can be calculatedfrom Expression (13). On the other hand, if the transmission mode isswitched from the second mode to either SFBC or the first mode, theinitial value for GINR_Adj can be calculated from the average value ofGINR_Adj0 and GINR_Adj1, as shown by Expression (14). There is no changefor GINR_Adj with the transmission mode switching between SFBC and thefirst mode.

GINR_Adj0=GINR_Adj1=GINR_Adj  Expression (13)

GINR_Adj=(GINR_Adj0+GINR_Adj1)/2  Expression (14)

After obtaining GINR values from the above expressions, these can beused to calculate the corresponding SINR values by adding DL PDSCHsignal PSD (power spectrum density). The TP values can be derived fromSINR by a table look-up method. The SINR to TP mapping table can bederived from computer simulation and field measurements.

Thus, in an example embodiment the controller 40 performs a switchbetween the plural transmission modes and, after the switch, begins apost-switch transmission mode using a throughput value acquired from thepre-switch transmission mode. For example, when a pre-switchtransmission mode uses only one adjusted GINR value for either onecodeword or two codewords and a post-switch transmission mode uses twoadjusted GINR values for two codewords, after the switch a post-switchtransmission mode is begun using the one adjusted GINR value acquiredfrom the pre-switch transmission mode as both of the two adjusted GINRvalues for the post-switch transmission mode. An another example, whenthe pre-switch transmission mode uses two adjusted GINR values for twocodewords, and the post-switch transmission mode uses one adjusted GINRvalue for either one codeword or two codewords, the two adjusted GINRvalues are averaged to obtain the one adjusted GINR value for thepost-switch transmission mode.

From the foregoing it can be seen that in the first mode of the exampleembodiment of FIG. 4 the transmit switch controller 40 uses a firstprecoding matrix indicator (PMI₁) to determine a first mode/first PMIthroughput value (TPV₁) and a second precoding matrix indicator (PMI₂)to determine a first mode/second PMI throughput value (TPV₂). If, afterperforming basic act (1) of FIG. 3, the terminal speed value (TSV) doesnot exceed the terminal speed threshold (as determined by act 4-4(Z),and if the first mode/first PMI throughput value (TPV₁) used as thefirst mode throughput value does not exceed a first throughput threshold(T2), the transmit switch controller 40 proceeds to perform other firstmode acts by essentially repeating basic acts (1) and (2) of FIG. 3, butusing the first mode/second PMI throughput value (TPV₂) as the firstmode throughput value. Then transmit switch controller 40 selects, basedon magnitude, either the first mode/first PMI throughput value (TPV₁) orthe first mode/second PMI throughput value (TPV₂) as a chosen throughputvalue. Then transmit switch controller 40 essentially repeats basic act(1) and basic act (2) of FIG. 3 using as the chosen throughput value forbasic act (2) of FIG. 3.

From the foregoing it can be seen that, in an example embodiment of thesecond mode, transmit switch controller 40 is configured to use a firstprecoding matrix indicator (PMI₁) to determine a second mode/first PMIthroughput value (TPV₁) and to use a second precoding matrix indicator(PMI₂) to determine a second mode/second PMI throughput value (TPV₂). Inthe second mode transmit switch controller 40 is further configured toperform certain other acts if both (a) the terminal speed value (TSV)does exceed the terminal speed threshold (T2) and (b) the secondmode/first PMI throughput value (TPV₁) does exceed a first throughputthreshold (T2). In particular, in such case the controller is configuredto repeat basic act (3) and basic (4) but using the second mode/secondPMI throughput value (TPV₂) as the second mode throughput value forbasic act (4). The transmit switch controller 40 is further configuredto select, based on magnitude, either the second mode/first PMIthroughput value (TPV₁) or the second mode/second PMI throughput value(TPV₂). Thereafter the controller repeats basic act (3) and basic (4)using as the chosen throughput value for basic act (4).

Further to the foregoing, in an example implementation of the secondmode, the controller 40 obtains feedback for both a first codeword and asecond codeword transmitted on the downlink (as shown by act 7-5). Thetransmit switch controller 40 determines the second mode/first PMIthroughput value (TPV₁) from (i) a second mode/first PMI/first codewordthroughput value (TPV₁₀) and (ii) a second mode/first PMI/secondcodeword throughput value (TPV₁₁), as reflected by act 5-2(C). Thetransmit switch controller 40 also determines the second mode/second PMIthroughput value (TPV2) from (iii) a second mode/second PMI/firstcodeword throughput value (TPV₂₀) and (iv) a second mode/secondPMI/second codeword throughput value (TPV₂₁), as reflected by act5-2(D). During the second mode the transmit switch controller 40 adjustsvalues of at least one of (i), (ii), (iii) and (iv) in accordance withfeedback for the respective codeword (see act 7-7).

It was mentioned above that the plural MIMO modes, e.g., of FIG. 2, mayalso include other modes. As an example, for a situation in which theRank Indication (RI) is 1, in another example embodiment the transmitswitch controller 40 may switch to a different PMI as defined in the3GPP standard of reference [1] rather than switch to space frequencyblock code (SFBC), and thereby perhaps obtain better performance thanspace frequency block code (SFBC).

As used herein, “terminal” or “wireless terminal” or “user equipment(UE)” may be a mobile station such as a mobile telephone or “cellular”telephone or a laptop with wireless capability, e.g., mobiletermination, and thus may be, for example, a portable, pocket,hand-held, computer-included, or car-mounted mobile device whichcommunicates voice and/or data via a radio access network. Moreover, aterminal or wireless terminal or UE may be a fixed terminal whichcommunicates voice and/or data via a radio access network.

In an example embodiment and as depicted by way of example in FIG. 1Band FIG. 2, the controller 36, and indeed transmit switch controller 40,may be realized by a machine platform. To this end FIG. 1B and FIG. 2employs a broken line to represent machine platform 90 which comprisescontroller 36 and transmit switch controller 40. The terminology“machine platform” is a way of describing how the functional units ofnode 28 can be implemented or realized by machine. The machine platform90 can take any of several forms, such as (for example) electroniccircuitry in the form of a computer implementation platform or ahardware circuit platform. A computer implementation of the machineplatform may be realized by or implemented as one or more computerprocessors or controllers as those terms are herein expansively defined,and which may execute instructions stored on non-transientcomputer-readable storage media. In such a computer implementation themachine platform 90 may comprise, in addition to a processor(s), amemory section (which in turn can comprise random access memory; readonly memory; an application memory (a non-transitory computer readablemedium which stores, e.g., coded non instructions which can be executedby the processor to perform acts described herein); and any other memorysuch as cache memory, for example). Another example platform suitablefor transmit switch controller 40 is that of a hardware circuit, e.g.,an application specific integrated circuit (ASIC) wherein circuitelements are structured and operated to perform the various actsdescribed herein. Similarly, the wireless terminal controller 46 ofwireless terminal 30 may be realized by machine platform 91.

In prior art technology as illustrated in FIG. 9A, the base station nodemade the decision and selection of transmission mode and communicatedthis decision to the wireless terminal through higher layer signalling.If the transmission mode is closed-loop MIMO, e.g., TM4, the wirelessterminal needs to report PMI, RI, and CQIs for two codewords. If thetransmission mode is open-loop MIMO, the wireless terminal does notreport PMI but reports CQI averaged over two codewords. Based on thewireless terminal report(s), the base station node decides themodulation and coding scheme for each codeword.

In contrast to prior art technology, the example embodiments of thetechnology disclosed herein such as illustrated in FIG. 9B achieve thesame advantage of closed loop MIMO while essentially operating in openloop MIMO fashion. Accordingly, as shown in FIG. 9B, the wirelessterminal of the technology disclosed herein only needs to report RI andan averaged CQI (e.g., CQI averaged over two codewords). Notably, thewireless terminal need not provide or transmit the Precoder MatrixIndicators (PMIs), Act B-2 shows the base station node estimating orother wise obtaining the terminal speed value (e.g., a Doppler shiftvalue) and, based on the report(s) from the wireless terminal,estimating the throughput value. As act B-3 the transmission modeselector 40 of base station node makes its transmission mode decision ordetermination based on the estimated throughput value and the terminalspeed value. An example manner in which the throughput value isestimated is understood with reference to FIG. 7, which culminates inact 7-8 as previously described. The transmission mode selector 40 mayalso consider the rank indicator (RI) provided by the wireless terminal.Then, as act B-4, the base station node communicates the transmissionmode decision to the wireless terminal using channelcondition-contemporaneous physical layer signaling and implements thetransmission mode decision. As act B-5 the wireless terminal implementsthe transmission mode decided by the base station node, and does notneed to send an acknowledgement to the base station because the basestation node has already implemented the transmission mode decision andhas communicated same using the physical layer signaling, so that thedecision signaling occurs at essentially the same time as the user data.

As mentioned above, as act B-4 the base station node communicates itstransmission mode decision to the wireless terminal using channelcondition-contemporaneous physical layer signaling. By being in the“physical layer” the decision signaling has a relation to the user datathat is being transmitted essentially contemporaneously with thephysical signaling, rather than being sent as pure, higher layersignaling as in the conventional case of FIG. 9A. The “relation” to theuser data means that the decision signaling is in a same or essentiallycontemporaneous time interval as the user data transport block for whichthe physical data is to be decoded. A transport block is understood tobe a prescribed set of resource elements defined by symbols andfrequency carriers. Thus, for example, to be “contemporaneous” the userdata may be carried in a PDSCH channel of a transport block, while thephysical signaling including the transmission mode decision may becarried in a PDCCH channel of the same time interval. But it should beunderstood that the physical signaling including the transmission modedecision does not have to be carried in the same time interval as theuser data, but may be carried in another time interval so long asanother physical layer signal is transmitted at a time in which thechannel conditions are essentially the same as the user data-carryingtransport block to which the signaling has relation. This ability of thephysical layer signaling including the transmission mode decision to beincluded in another transmission unit, frame, block or interval (e.g.,transmission time interval) that experiences essentially the samechannel condition is what is meant by “channel condition-contemporaneousphysical layer signaling”. In other words, the decision/determination oftransmission mode is communicated in the physical layer at a point intime that is close enough in time to the user data to which it appliesso that essentially the same channel conditions are applicable (e.g.,the channel conditions have not appreciably changed) for both thesignaling and the user data. Communication of the transmission modedecision using channel condition-contemporaneous physical layersignaling rather than higher layer signaling provides more efficiencyby, e.g. avoiding delay and additional overhead (e.g., resourceoverhead) that attends the higher layer signaling.

In essence, the open loop MIMO techniques employed by exampleembodiments of the technology disclosed herein, which base thetransmission mode decision on throughput value and terminal speed value,essentially provide results comparable to those of a closed loop MIMOtechnique. This is advantageous since closed loop MIMO generallyprovides greater throughput. But the closed loop MIMO techniques requirecommunication from the wireless terminal to the base station node of thePrecoder Matrix Indicators (PMIs), and accordingly require greaterutilization of transmission resources and delay to accommodate thecommunication of this information. Moreover, in conventional closed loopMIMO techniques the base station node uses the PMI proposed by thewireless terminal. But example embodiments of the technology disclosedherein the transmission mode selector 40 essentially considers, based onthroughput value, all transmission constellation options and therebyessentially determines which PMI should be utilized without PMI(s)having to be communicated by the wireless terminal.

Whereas in closed loop MIMO there are at least three constellationscorresponding to different PMIs that must be decided by the wirelessterminal, the technology disclosed herein by using open loop MIMO hasthe base station node look at all constellation options and chooses thebest PMI that the node can perceive, which is essentially the equivalentof a closed loop MIMO technique, without the overhead of closed loopMIMO (which involved the wireless terminal sending the PMI(s), the CQIsof two codewords, etc.). In so doing, the transmit switch controller 40calculates the throughput value, and continually checks whether thethroughput value based on a first PMI (PMI₁) or a second PMI (PMI₂) isbetter. In the prior art this type of comparison between PMIs would havebeen performed by the wireless terminal, and communication of results ofsuch comparison would be reported as needed to the network node, therebyutilizing additional overhead signaling (which is not required by thetechnology disclosed herein).

A problem with closed loop MIMO is that the wireless terminal reportsthe channel conditions (CQIs, PMI, etc.) as it perceived them on thelast grant (of resources), which may be several milliseconds previouslyand therefore may be out-dated. The techniques of the technologydisclosed herein, on the other hand, are much more current, e.g., moretimely.

For traditional open loop MIMO the PMI is fixed, whether it be a good orbad value. However, for the open loop MIMO techniques of exampleembodiments of the technology disclosed herein, the transmission modeselector 40 is able to determine a transmission mode based on a bestPMI, using the estimated throughput value and terminal speed value.

Thus, as stated above, the open loop MIMO techniques employed by exampleembodiments of the technology disclosed herein, which base thetransmission mode decision on throughput value and terminal speed value,essentially provide results comparable to those of a closed loop MIMOtechnique, and advantageously provide greater throughput without theincreased overhead and complexity of closed loop MIMO operation.

The technology disclosed herein provides numerous advantages. Anon-exhaustive list of example advantages include the following:

-   -   Reduction in the working load of a wireless terminal because the        technology removes the need for a wireless terminal to calculate        the Precoding Matrix Indicator (PMI).    -   Reduction in the uplink control channel overhead because no PMI        reports are required.    -   Usage of same CQI report format and method for both OL-MIMO with        large CDD and OL-MIMO without CDD. Only one CQI (averaged over        two code words) is reported, thus reducing the uplink control        channel overhead further.    -   Reduction of the open loop to closed loop switch time and        simplification of the higher layer signaling (RRC) for transmit        mode switching.    -   Increasing the open loop performance by using more than one PMI.        The outer loop measurement for the previous transmission mode        may be re-used for the subsequent transmission mode which will        avoid producing an outer-loop transient step.    -   Usage of Doppler and throughput measurements to switch between        the MIMO modes, which improves robustness and performance.

The technology disclosed herein advantageously uses use an extremelylimited feedback (e.g. CQI and terminal speed value) received from thewireless terminal by the eNB in order to enable the eNB to derive theinformation usually communicated in CL-MIMO, thereby allowing the eNB tooperate in OL-MIMO and benefit from all of the advantages of CL-MIMOwithout the inconvenience of signaling and use of UL signalingresources, which in turn enables the eNB to have a higher density ofwireless terminals operating in MIMO. The technology disclosed hereinadvantageously provides an implicit closed loop for OL-MIMO which is insharp contrast to the explicit CL-MIMO or non-minimal limited feedback.

One or more of the following references, all of which are incorporatedherein by reference, may be pertinent to one or more aspects of thetechnology disclosed herein:

[1] 3GPP TS 36.211: “Evolved Universal Terrestrial Radio Access(E-UTRA); Physical channels and modulation”.[2] 3GPP TS 36.212: “Evolved Universal Terrestrial Radio Access(E-UTRA); Multiplexing and channel coding”3GPP TS 36.213: “Evolved Universal Terrestrial Radio Access (E-UTRA);Physical layer procedures”.

Although the description above contains many specificities, these shouldnot be construed as limiting the scope of the technology disclosedherein but as merely providing illustrations of some of the presentlypreferred embodiments of the technology disclosed herein. Thus the scopeof the technology disclosed herein should be determined by the appendedclaims and their legal equivalents. Therefore, it will be appreciatedthat the scope of the technology disclosed herein fully encompassesother embodiments which may become obvious to those skilled in the art,and that the scope of the technology disclosed herein is accordingly tobe limited by nothing other than the appended claims, in which referenceto an element in the singular is not intended to mean “one and only one”unless explicitly so stated, but rather “one or more.” All structural,mechanical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the technology disclosed herein, for it to beencompassed by the present claims. Furthermore, no element, component,or method step in the present disclosure is intended to be dedicated tothe public regardless of whether the element, component, or method stepis explicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

What is claimed is:
 1. A method of operating a node of a radio accessnetwork which communicates over a radio interface with a wirelessterminal, the method comprising: the node obtaining with respect to thewireless terminal both a terminal speed value and a throughput value;the node using both the terminal speed value and the throughput value tomake a determination when to switch between plural transmission modes,at least one of the transmission modes comprising a multiple inputmultiple output (MIMO) mode.
 2. The method of claim 1, furthercomprising the node using both the terminal speed value and thethroughput value to make a determination when to switch between pluralmultiple input multiple output (MIMO) modes for communicating indownlink transmission with the wireless terminal, the plural MIMO modescomprising a first mode wherein open loop MIMO operates with cyclicaldiversity delay and a second mode wherein open loop MIMO operateswithout cyclical diversity delay
 3. The method of claim 2, wherein thenode determines in which of the plural modes the node is to operate withrespect to the wireless terminal by performing the following acts: (1)if the node is in the first mode and the terminal speed value exceeds aterminal speed threshold, keeping the downlink transmission in the firstmode; (2) if the node is in the first mode and the terminal speed valuedoes not exceed the terminal speed threshold: (2a) switching thedownlink transmission to the second mode when a first mode throughputvalue exceeds a throughput threshold; and (2b) keeping the downlinktransmission in the first mode when the first mode throughput value doesnot exceed the throughput threshold; (3) if the node is in the secondmode and the terminal speed value exceeds the terminal speed threshold,switching the downlink transmission to the first mode; and (4) if thenode is in the second mode and the terminal speed value does not exceedthe terminal speed threshold: (4a) switching the downlink transmissionto the first mode when a second mode throughput value does not exceedthe throughput threshold; and (4b) keeping the downlink transmission inthe second mode when the second mode throughput value does exceed thethroughput threshold.
 4. The method of claim 3, further comprising:using a first precoding matrix indicator to determine a first mode/firstPMI throughput value which is used for the first mode throughput valuewhen performing acts (1) and (2); using a second precoding matrixindicator to determine a first mode/second PMI throughput value which isused for the first mode throughput value then performing acts (1) and(2); if the first mode/first PMI throughput value exceeds the firstmode/second PMI throughput value, using the first mode/first PMIthroughput value as the first mode throughput value when repeating acts(1) and (2); if the first mode/first PMI throughput value does notexceed the first mode/second PMI throughput value, using the firstmode/second PMI throughput value as the first mode throughput value whenrepeating acts (1) and (2).
 5. The method of claim 4, wherein feedbackis obtained for both a first codeword and a second codeword transmittedon the downlink, wherein at least one of the first mode/first PMIthroughput value and the first mode/second PMI throughput value isdetermined by the sum of the first codeword throughput value and thesecond codeword throughput value.
 6. The method of claim 3, furthercomprising: using a first precoding matrix indicator to determine asecond mode/first PMI throughput value which is used for the second modethroughput value when performing act (3); using a second precodingmatrix indicator to determine a second mode/second PMI throughput valuewhich is used for the second mode throughput value when performing act(3); the node performing the following further acts when in second modeand when the terminal speed value does not exceed the terminal speedthreshold: comparing a maximum of the second mode/first PMI throughputvalue and the second mode/second PMI throughput value to the throughputthreshold; if the maximum of the second mode/first PMI throughput valueand the second mode/second PMI throughput value does not exceed thethroughput threshold, switching the downlink transmission to the firstmode; otherwise: when the maximum of the second mode/first PMIthroughput value and the second mode/second PMI throughput value is thesecond mode/first PMI throughput value, repeating act (3) using thesecond mode/first PMI throughput value as the second mode throughputvalue; or when the maximum of the second mode/first PMI throughput valueand the second mode/second PMI throughput value is the secondmode/second PMI throughput value, repeating act (3) using the secondmode/second PMI throughput value as the second mode throughput value. 7.The method of claim 6, wherein feedback is obtained for both a firstcodeword and a second codeword transmitted on the downlink, and furthercomprising: determining the second mode/first PMI throughput value from(i) a second mode/first PMI/first codeword throughput value and (ii) asecond mode/first PMI/second codeword throughput value; determining thesecond mode/second PMI throughput value from (iii) a second mode/secondPMI/first codeword throughput value and (iv) a second mode/secondPMI/second codeword throughput value; adjusting values of at least oneof (i), (ii), (iii) and (iv) in accordance with feedback for therespective codeword.
 8. The method of claim 1, further comprisingestimating the throughput value by: (1) using a channel qualityindication reported by the wireless terminal to determine a signal tointerference plus noise ratio (SINR) for each codeword; (2) deriving achannel gain to interference noise ratio (GINR) from the SINR bysubtracting a power spectrum density of a reference signal (PSD_(RS));(3) applying a smoothing filter to the GINR to obtain a smoothed GINRvalue; (4) using HARQ ACK/NACK feedback to adjust the GINR to obtain anadjusted GINR value; (5) using the smoothed GINR value, the adjustedGINR value, and a power spectrum density of a PDSCH signal to obtain thethroughput value for each codeword.
 9. The method of claim 8, furthercomprising adding the throughput value for a first codeword and thethroughput value for a second codeword to obtain a final throughputvalue for the multiple input multiple output (MIMO) mode.
 10. The methodof claim 8, wherein the GINR adjusted value for the first mode isdetermined by: adding twice an up-step value to a previous GINR adjustedvalue if the feedback for both a first codeword and a second codeword isa positive acknowledgement; subtracting twice a down-step value from theprevious GINR adjusted value if the feedback for both the first codewordand the second codeword is a negative acknowledgement; adding adifference between the up-step value and the down-step value to theprevious GINR adjusted value if the feedback for one codeword is apositive acknowledgement and the feedback for the other codeword is anegative acknowledgement.
 11. The method of claim 8, wherein the GINRadjusted value of each codeword for the second mode is determined by:adding an up-step value to a previous GINR adjusted value of a firstcodeword if the feedback for the first codeword is a positiveacknowledgement; adding an up-step value to a previous GINR adjustedvalue of a second codeword if the feedback for the second codeword is apositive acknowledgement; subtracting a down-step value from theprevious GINR adjusted value of the first codeword if the feedback forthe first codeword is a negative acknowledgement; and subtracting adown-step value from the previous GINR adjusted value of the secondcodeword if the feedback for the second codeword is a negativeacknowledgement.
 12. The method of claim 8, wherein the GINR adjustedvalue for a third mode is determined by: adding an up-step value to aprevious GINR adjusted value if the feedback is a positiveacknowledgement; subtracting a down-step value from the previous GINRadjusted value if the feedback is a negative acknowledgement.
 13. Themethod of claim 8, further comprising performing a switch between theplural transmission modes and, after the switch, beginning a post-switchtransmission mode using the adjusted GINR acquired from the pre-switchtransmission mode.
 14. The method of claim 8, further comprising: when apre-switch transmission mode uses only one adjusted GINR value foreither one codeword or two codewords and a post-switch transmission modeuses two adjusted GINR values for two codewords, after the switchbeginning a post-switch transmission mode using the one adjusted GINRvalue acquired from the pre-switch transmission mode as both of the twoadjusted GINR values for the post-switch transmission mode; when thepre-switch transmission mode uses two adjusted GINR values for twocodewords, and the post-switch transmission mode uses one adjusted GINRvalue for either one codeword or two codewords, averaging the twoadjusted GINR values to obtain the one adjusted GINR value for thepost-switch transmission mode.
 15. The method of claim 1, wherein theplural transmission modes further comprises a third mode which is a rankone mode, and wherein the method further comprises the node using a rankindicator value and/or the terminal speed value to make thedetermination when to switch the downlink transmission to or away fromthe rank one mode.
 16. The method of claim 1, further comprising thenode using HARQ feedback to adjust the throughput value.
 17. The methodof claim 1, further comprising the node communicating the determinationto the wireless terminal using channel condition-contemporaneousphysical layer signaling.
 18. A node of a radio access network whichcommunicates over a radio interface with a wireless terminal, the nodecomprising: a transmitter configured to selectively operate in at leasta multiple input multiple output (MIMO) mode for downlink transmissionover the radio interface; a controller configured to use both a terminalspeed value and a throughput value to make a determination when toswitch between plural transmit modes, the plural transmit modesincluding at least one multiple input multiple output (MIMO) mode. 19.The node of claim 18, wherein the controller is configured to use boththe terminal speed value and the throughput value to make adetermination when to switch between plural multiple input multipleoutput (MIMO) modes for communicating with the wireless terminal, theplural MIMO modes comprising a first mode wherein open loop MIMOoperates with cyclical diversity delay and a second mode wherein openloop MIMO operates without cyclical diversity delay.
 20. The node ofclaim 19, wherein the controller is configured to determine in which ofthe plural modes the node is to operate with respect to the wirelessterminal by performing the following acts: (1) if the node is in thefirst mode and the terminal speed value exceeds a terminal speedthreshold, keeping the downlink transmission in the first mode; (2) ifthe node is in the first mode and the terminal speed value does notexceed the terminal speed threshold: (2a) switching the downlinktransmission to the second mode when a first mode throughput valueexceeds a throughput threshold; and (2b) keeping the downlinktransmission in the first mode when the first mode throughput value doesnot exceed the throughput threshold; (3) if the node is in the secondmode and the terminal speed value exceeds the terminal speed threshold,switching the downlink transmission to the first mode; and (4) if thenode is in the second mode and the terminal speed value does not exceedthe terminal speed threshold: (4a) switching the downlink transmissionto the first mode when a second mode throughput value does not exceedthe throughput threshold; and (4b) keeping the downlink transmission inthe second mode when the second mode throughput value does exceed thethroughput threshold.
 21. The node of claim 20, wherein the controlleris configured to: use a first precoding matrix indicator to determine afirst mode/first PMI throughput value which is used for the first modethroughput value when performing acts (1) and (2); use a secondprecoding matrix indicator to determine a first mode/second PMIthroughput value which is used for the first mode throughput value thenperforming acts (1) and (2); if the first mode/first PMI throughputvalue exceeds the first mode/second PMI throughput value, use the firstmode/first PMI throughput value as the first mode throughput value whenrepeating acts (1) and (2); if the first mode/first PMI throughput valuedoes not exceed the first mode/second PMI throughput value, use thefirst mode/second PMI throughput value as the first mode throughputvalue when repeating acts (1) and (2).
 22. The node of claim 21, whereinfeedback is obtained for both a first codeword and a second codewordtransmitted on the downlink, and wherein the controller is configured todetermine at least one of the first mode/first PMI throughput value andthe first mode/second PMI throughput value by the sum of the firstcodeword throughput value and the second codeword throughput value. 23.The node of claim 20, wherein the controller is configured to: use afirst precoding matrix indicator to determine a second mode/first PMIthroughput value which is used for the second mode throughput value whenperforming act (3); use a second precoding matrix indicator to determinea second mode/second PMI throughput value which is used for the secondmode throughput value when performing act (3); wherein the controller isconfigured to perform the following further acts when in second mode andwhen the terminal speed value does not exceeds the terminal speedthreshold: compare a maximum of the second mode/first PMI throughputvalue and the second mode/second PMI throughput value to the throughputthreshold; if the maximum of the second mode/first PMI throughput valueand the second mode/second PMI throughput value does not exceed thethroughput threshold, switch the downlink transmission to the firstmode; otherwise: when the maximum of the second mode/first PMIthroughput value and the second mode/second PMI throughput value is thesecond mode/first PMI throughput value, repeat act (3) using the secondmode/first PMI throughput value as the second mode throughput value; orwhen the maximum of the second mode/first PMI throughput value and thesecond mode/second PMI throughput value is the second mode/second PMIthroughput value, repeat act (3) using the second mode/second PMIthroughput value as the second mode throughput value.
 24. The node ofclaim 23, wherein the controller is configured to: obtain feedback forboth a first codeword and a second codeword transmitted on the downlink;determine the second mode/first PMI throughput value from (i) a secondmode/first PMI/first codeword throughput value and (ii) a secondmode/first PMI/second codeword throughput value; determine the secondmode/second PMI throughput value from (iii) a second mode/secondPMI/first codeword throughput value and (iv) a second mode/secondPMI/second codeword throughput value; adjust values of at least one of(i), (ii), (iii) and (iv) in accordance with feedback for the respectivecodeword.
 25. The node of claim 18, wherein the controller is configuredto estimate the throughput value by: (1) using a channel qualityindication reported by the wireless terminal to determine a signal tointerference plus noise ratio (SINR); (2) deriving a channel gain tointerference noise ratio (GINR) from the SINR by subtracting a powerspectrum density of a reference signal (PSD_(RS)); (3) applying asmoothing filter to the GINR to obtain a smoothed GINR value; (4) usingHARQ ACK/NACK feedback to adjust the GINR to obtain an adjusted GINR;(5) using the smoothed GINR value, the adjusted GINR value, and a powerspectrum density of a PDSCH signal to obtain the throughput value foreach codeword.
 26. The node of claim 25, wherein the controller isconfigured to add the throughput value for a first codeword and a secondcodeword to obtain a final throughput value for the multiple inputmultiple output (MIMO) mode.
 27. The node of claim 25, wherein thecontroller is configured to determine the GINR adjusted value for thefirst mode by: adding twice an up-step value to a previous GINR adjustedvalue if the feedback for both a first codeword and a second codeword isa positive acknowledgement; subtracting twice a down-step value from theprevious GINR adjusted value if the feedback for both the first codewordand the second codeword is a negative acknowledgement; adding adifference between the up-step value and the down-step value to theprevious GINR adjusted value if the feedback for one codeword is apositive acknowledgement and the feedback for the other codeword is anegative acknowledgement.
 28. The node of claim 25, wherein thecontroller is configured to determine the GINR adjusted value of eachcodeword for the second mode by: adding an up-step value to a previousGINR adjusted value of a first codeword if the feedback for the firstcodeword is a positive acknowledgement; adding an up-step value to aprevious GINR adjusted value of a second codeword if the feedback forthe second codeword is a positive acknowledgement; subtracting adown-step value from the previous GINR adjusted value of the firstcodeword if the feedback for the first codeword is a negativeacknowledgement; and subtracting a down-step value from the previousGINR adjusted value of the second codeword if the feedback for thesecond codeword is a negative acknowledgement.
 29. The node of claim 25,wherein the controller is configured to determine the GINR adjustedvalue for a third mode by: adding an up-step value to a previous GINRadjusted value if the feedback is a positive acknowledgement;subtracting a down-step value from the previous GINR adjusted value ifthe feedback is a negative acknowledgement.
 30. The node of claim 25,wherein when the control performs a switch between the pluraltransmission modes and, after the switch, the controller is configuredto begin a post-switch transmission mode using the adjusted GINRacquired from the pre-switch transmission mode.
 31. The node of claim25, wherein the controller is configured to begin the post-switchtransmission mode using the adjusted GINR acquired from the pre-switchtransmission mode by: when a pre-switch transmission mode uses only oneadjusted GINR value for either one codeword or two codewords and apost-switch transmission mode uses two adjusted GINR values for twocodewords, after the switch beginning a post-switch transmission modeusing the one adjusted GINR value acquired from the pre-switchtransmission mode as both of the two adjusted GINR values for thepost-switch transmission mode; when the pre-switch transmission modeuses two adjusted GINR values for two codewords, and the post-switchtransmission mode uses one adjusted GINR value for either one codewordor two codewords, averaging the two adjusted GINR values to obtain theone adjusted GINR value for the post-switch transmission mode.
 32. Thenode of claim 18, wherein the plural transmission modes furthercomprises a third mode which is a rank one mode, and wherein the node isfurther configured to use a rank indicator value and/or the terminalspeed value to make the determination when to switch the downlinktransmission to or away from the rank one mode.
 33. The node of claim18, wherein the controller is configured to use HARQ feedback to adjustthe throughput value.
 34. The node of claim 18, wherein the transmittercommunicates the determination to the wireless terminal using channelcondition-contemporaneous physical layer signaling.